Why do I sometimes sit up with a snort when I fall asleep on airplane? Position?

Yes. Your soft palate may close against the back wall of your throat closing off air flow. This will cause you to awaken with a sudden snort as you generate enough pressure to overcome the obstruction. This is the same physiology that causes obstructive sleep apnea, so if you snore at home you might need a sleep study to assess for obstructive sleep apnea.
Startle Response. When falling asleep in the sitting position we often droop our heads. This can cause a startle response which can be accompanied by a small snort. Others bend their heads forward enough to cause a temporary narrowing of the airway.Your body 'objects' to this and causes you wake up and inhale suddenly also causing a small snort. A "u" shaped travel pillow can help position you more comfortably.